The present invention relates to a method for adjusting a parameter of a power device by changing an input signal to said power device, and also to a corresponding device. In particular, the present invention is directed to a method and corresponding device for compensating a performance change of a transistor by adjusting a bias point.
Power devices using for example semiconductor elements such as field effect transistors (FET) or metal oxide silicon (MOS) FET""s are widely spread in various technology fields. Said power devices are designed for power amplification and able to handle high voltages and large currents. A major problem in the usage of such power devices mentioned above is to keep the performance such as linearity, amplifier gain, output power and the like e.g. of the used semiconductor element constant over a long period of time. This is in particular important for variable envelope signals.
Due to thermal effects (e.g. self heating or external heating), aging and the like, characteristic parameters of the transistor can be changed. Therefore, a constant performance, in particular the linearity of the semiconductor element, can not be guaranteed over a long period of time. However, for an effective usage of the power device, a constant performance (e.g. linearity) is extremely important.
Therefore, hitherto several solutions are proposed to keep the performance of such a power device, i.e. of the semiconductor element such as a MOSFET transistor, as stable as possible in time. One conventional solution is to detect the drain current ID through a resistor connected in series. A bias controlling is then performed on the basis of this detected current. However, due to the insertion of the resistor it is possible that in the case of a high power the drain voltage can be significantly changed. Moreover, any resistive element introduced in the path may decrease the efficiency of the amplifier.
Another known solution is to separate a cell of the transistor structure and use a signal measured therefrom as a DC reference to compensate a drain current change. However, this can be prepared only by the element manufacturer since this solution requires a new element architecture.
Therefore, it is an object of the present invention to provide a method and also a corresponding device for controlling an output signal of a power device by changing an input signal to said power device which are free from the above mentioned drawbacks.
According to the present invention, this object is achieved by a method for adjusting a parameter of a power device by changing an input signal to said power device, said method comprising the steps of taking a sample of an output signal of said power device, detecting at least a first and a second signal component level of said sampled output signal, processing the at least detected first and second signal component level, deciding on the basis of a result obtained in said processing step, whether a compensation of a change of said parameter is required, and if such a change is to be compensated, determining a changing value for said input signal and adjusting the parameter by changing said input signal by said determined changing value.
Furthermore, the present invention proposes a device for adjusting a parameter of a power device by changing an input signal to said power device, said device comprising a coupler means for taking a sample of an output signal of said power device, at least two detector means, respectively, for detecting at least a first and a second signal component level of said sampled output signal, and a feedback circuit adapted to process the at least detected first and second signal component level, to decide on the basis of a result obtained in said processing, whether a compensation of a change of said parameter is required, and if such a change is to be compensated, to determine a changing value for said input signal and to adjust the parameter by changing said input signal by said determined changing value.
According to the present invention, the proposed method and/or device require no change in the element architecture. Also, an element such as a resistor is not needed within the network, therefore the risk of further changes of performance, for example at high power, is prevented.
Moreover, the present invention provides a circuit which does almost consume no RF power and which is easy to implement. Furthermore, it is possible to compensate drift effects of e.g. a drain current over a long period of time, for example due to aging, but also short time changes, for example due to a quick temperature change of the semiconductor element. As the control is related directly to the linearity of the semiconductor element (i.e. transistor), a bias controlling method according to the invention works very accurate. In particular, as no interruption is required for measurement or adjusting the present invention provides an effective control of the power device.
Preferred embodiments of the invention are described herein below in detail by way of example with reference to the accompanying drawings.